Generally, video display systems based on spatial light modulators (SLMs) are increasingly being used as an alternative to display systems using cathode ray tubes (CRTs). SLM systems generally provide high resolution displays without the bulk and power consumption of CRT systems.
One type of SLM is a digital micromirror device (DMD). Digital micromirror devices have also been called deformable micromirror devices, although that term generally is now used to describe devices that operate in an analog mode. DMDs may be used for either direct-view or rear or front projection display applications. A DMD has an array of micro-mechanical display elements, each having a tiny mirror that is individually addressable by an electronic signal. Depending on the state of its addressing signal, each mirror tilts so that it either does or does not reflect light to the image plane. The mirrors may be generally referred to as “display elements,” which correspond to the pixels of the image that they generate. Generally, displaying pixel data is accomplished by loading memory cells connected to the display elements. After a display element's memory cell is loaded, the display element is reset so that it tilts in the on or off position represented by the new data in the memory cell. The display elements are able to maintain the on or off state for controlled display times.
In the prior art, a reset sequence generally is used to drive a DMD pixel. The reset sequence generally comprises 5 steps including (1) memory write, (2) reset, (3) release and differentiation, (4) landing and latch, and (5) stabilization. This cycle, including mechanical switching time, generally takes about 15 μsec to complete. While a bias voltage is applied, the micromirror is electro-mechanically latched, allowing update of memory data for the next reset sequence.
One potential disadvantage with the mechanical nature of a DMD mirror is that a transient response typically is seen just after the mirror tip (spring tip) lands on a landing plate. Then the response gradually decays in accordance with a receiving resistance from ambient gas at about one atmosphere pressure. The resonant vibration seen in a transient response generally originates from the physical structure of the micromirror. A transient frequency of the mirror may be about 450-550 kHz, although the value may vary depending on the specific application. Data loading to an SRAM under the micromirror generally is performed after the response declines to a level that is low enough to guarantee the prevention of a micromirror malfunction. Therefore a stabilization period is required to remove mechanical instability in the switching operation of a micromirror. This time may take up two-thirds of the mechanical switching time.
In general, the total response time is composed of the mechanical switching time and the data loading time for a mirror section, which time determines the minimum cycle time. This directly relates to an LSB time for binary pulse width modulation techniques in controlling gray levels, which generally determines the color bit depth on a DMD system.
Generally, bit depth requirements are increasing the need for shorter LSB times on SLM projectors. In the prior art, a fast clear technique has been used to achieve high bit depth. Generally, fast clear is a function whereby all mirrors are sent to a flat state by a single voltage change. Fast clear, however, may be reliable to only as low as about 14 μsec t-wait time (approximately 15 μsec cycle time). Moreover, even at that level, there may be a significant yield loss.
Alternatively, a reset-release function has been used in the prior art to achieve high bit depth. The reset-release technique can provide a shorter cycle time (e.g., 8 μsec) than the fast clear technique. A reset-release pulse is used to cause a display element to assume an unaddressed flat state, in which the display element “floats.” During this float time, the next address state is loaded. Then a bias value is reapplied and the display element assumes the new address state. The use of reset-release display times is described in U.S. Pat. No. 5,764,208 entitled “Reset Scheme for Spatial Light Modulators,” and U.S. Pat. No. 6,008,785, entitled “Generating Load/Reset Sequences for Spatial Light Modulators,” each of which patents is assigned to Texas Instruments Incorporated and incorporated herein by reference.
One potential disadvantage with the prior art reset-release pulse sequence is that it may introduce a significant loss in contrast (e.g., about 30%). In addition, it may introduce a potential residual image artifact with hinge memory over the device lifetime. These disadvantages generally are due to a significant overshoot of the mirror into the pupil of the optical system after launch.